Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F41/00—Apparatus or processes specially adapted for manufacturing or assembling magnets, inductances or transformers; Apparatus or processes specially adapted for manufacturing materials characterised by their magnetic properties
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/381—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets
  • G01R33/3815—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field using electromagnets with superconducting coils, e.g. power supply therefor
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/387—Compensation of inhomogeneities
  • G01R33/3873—Compensation of inhomogeneities using ferromagnetic bodies ; Passive shimming
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R33/00—Arrangements or instruments for measuring magnetic variables
  • G01R33/20—Arrangements or instruments for measuring magnetic variables involving magnetic resonance
  • G01R33/28—Details of apparatus provided for in groups G01R33/44 - G01R33/64
  • G01R33/38—Systems for generation, homogenisation or stabilisation of the main or gradient magnetic field
  • G01R33/387—Compensation of inhomogeneities
  • G01R33/3875—Compensation of inhomogeneities using correction coil assemblies, e.g. active shimming
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
  • H01F7/00—Magnets

Definitions

• This invention relates to a unitary superconducting electromagnet and method of forming same, and more particularly to such a unitary superconducting electromagnet and method which utilizes superconducting coils and an outer ferromagnetic body about the coils in obtaining a desired shape and intensity of the magnetic field.
• a superconducting magnet which utilizes superconducting coils substantially decreases the power requirements and permits much larger current densities thus reducing the amount of conductor material required for a specific predetermined current.
• Some superconducting magnets have used magnetic iron for a shield for the magnetic field but these shielded magnets have been limited to non-precision magnetic fields having a magnetic field intensity not substantially greater than the magnetic saturation of the iron which is around 1.5 Tesla.
• Magnetic iron shields have been used heretofore for magnetic fields having an intensity not substantially greater than the magnetic saturation of iron, such as in U. S. Patent No. 4,646,045, dated February 24, 1987 which shows nuclear magnetic resonance (NMR) magnets in medical apparatus in order to minimize or eliminate any magnetic fringe or stray field which could interfere with adjacent equipment operation.
• NMR nuclear magnetic resonance
• a separate shield was utilized in U. S. Patent No. 4,646,045 with a minimum amount of ferromagnetic material in order to minimize the effect of the shield on the field homogeneity and there was no suggestion of having such a shield effective at magnetic intensities greater than the magnetic saturation of the magnetic iron in the shield, and particularly to increase the magnetic field intensity.
• a magnetic iron shield provides a part of the closed magnetic path and affects the shape and intensity of the magnetic field.
• the superconducting coils and shield be accurately positioned in relation to each other and to the magnetic field so that the desired shape and intensity of the magnetic field is obtained.
• such a superconducting electromagnet has not been provided in which the positioning of the coils and ferromagnetic shield in relation to each other and in relation to the magnetic field could be accurately predetermined so that a magnetic field uniformity of at least around one part per million is obtained, particularly when utilized in a magnetic field having an intensity substantially greater than the magnetic saturation of the iron.
• This invention is particularly directed to a unitary 5 electromagnet and method of forming same in which a unitary electromagnetic structure has an outer ferromagnetic body positioned accurately and precisely relative to an inner body containing superconducting coils to obtain in an efficient manner the desired shape and intensity of the
• At least two (2) coils are provided within the inner body and the wire is precisely wound on the core of each coil under
• the unitary electromagnet structure having an outer ferromagnetic body obtains a magnetic field uniformity of at least around one (1) part per million within a sphere of about one-quarter (1/4) of the diameter or transverse dimension of the coil aperture or opening,
• the outer ferromagnetic body of the unitary electromagnet adjacent the superconducting coils increases the intensity of the magnetic field as much as twenty (20) percent or more and this is accomplished only by an accurate calculation of the parameters of the magnetic field and the relative locations of the inner and outer bodies in relation to the coils and the magnetic field.
• the outer ferromagnetic body distributes the magnetic field flux in a smooth uniform manner at magnetic field intensities substantially above the magnetic saturation of magnetic iron as high as around ten (10) Tesla. Thus, the saturation of the magnetic iron in the outer body enhances the uniform distribution of flux.
• the volume, shape, and intensity of the magnetic field for the unitary superconducting electromagnetic structure are established and an initial design is created to satisfy these requirements in addition to providing some means for access to the magnetic field in order to place equipment and samples therein or to receive patients or animals for testing or treatment.
• the inner body containing the coils and the outer ferromagnetic body are positioned in relation to each other and to the magnetic field so that a desired effect on the intensity and shape of the magnetic field may be obtained with the precise adjustment of the inner body in relation to the outer body.
• An initial design is established based on (1) computer programs using (a) parameters of the volume, shape, and intensity of the desired magnetic field; and (b) accurate information concerning the mechanical, thermal, electrical, and magnetic properties of the materials proposed for the unitary electromagnetic structure, including particularly the thermal and magnetic properties of the iron for predicting precisely the permeability of the iron at all values of the magnetic field; (2) appropriate engineering techniques to obtain the precise positioning of the components of the magnet including particularly the positioning of the coils and the outer ferromagnetic body in relation to each other and to the magnetic field; and (3) utilization of appropriate electromagnetic equations for achieving a practical, economical, engineering design.
• the computer programs may incorporate, if desired, the appropriate engineering techniques and electromagnet equations set forth under items (2) and (3) above or these items may be employed separately from the computer programming in obtaining the initial design. Then, the prototype unitary superconducting electromagnetic structure is manufactured based on the results and calculations obtained.
• the magnetic iron increases the intensity of the magnetic field at least around ten (10) percent and as much as around twenty (20) percent or more due to the precise location of the magnetic iron with respect to the magnetic field and the coils.
• the outer magnetic iron body absorbs and redirects the magnetic fringe field to provide such an increase in the magnetic intensity.
• the accurate and precise positioning of the outer magnetic iron body relative to the inner body containing the coils within a tolerance of around one mil (0.001 inch) also results in the high degree of uniformity of the field of at least around one (1) part per million.
• the coils are wound within a precise tolerance in all directions and this is accomplished with a predetermined tensioning of the wire forming the coils.
• the electromagnet operates in a persistent current mode in which a superconducting switch switches from a charging power supply to the persistent mode. It may also be necessary to provide adequate cooling for the superconducting coils and helium or nitrogen may be employed for such cooling so that the superconducting members are cooled below their critical temperature to cause such members to become superconducting.
• An additional object of the invention is to provide such a superconducting electromagnet and method in which the outer magnetic iron body increases the magnetic intensity of the magnetic field at least around ten (10) percent and as much as twenty (20) percent or more.
• Another object of this invention is to provide a method for manufacture of such a unitary superconducting electromagnet in which a computer program and software are utilized which include accurate information concerning the mechanical, thermal, electrical, and magnetic properties of the materials used in the magnet, as well as appropriate electromagnetic equations and engineering techniques for positioning the coils and outer magnetic iron body in relation to each other within a tolerance of one (1) mil to obtain the desired intensity and shape of the magnetic field for maximum uniformity of the field.
• Figure 1 is a perspective with certain parts broken away of an example of a unitary superconducting electromagnet in accordance with the present invention having an inner body with a central bore therethrough and an outer ferromagnetic body about the superconducting coils of the inner body and means to position accurately the outer body relative to the coils of the inner body;
• Figure 2 is an end elevation of the electromagnet shown in Figure 1 showing the outer ferromagnetic body adjacent the coils formed in a plurality of segments and supported on a cradle type base support;
• Figure 3 is a transverse cross section of the generally cylindrical inner body for the coils of the electromagnet illustrating particularly the arrangement of heat shields and means for the flow of a cooling medium, such as helium, for cooling the superconducting materials;
• a cooling medium such as helium
• Figure 4 is a longitudinal sectional view taken generally along line 4-4 of Figure 2 and showing the inner cylindrical body for the coils mounted within the outer magnetic iron body about the inner body;
• Figure 5 is an enlarged longitudinal section of a fragment of Figure 4 showing specifically the arrangement of the magnetic coils within the inner cylindrical body;
• Figure 6 is a graph of a quadrant of the magnetic fringe field at a magnetic intensity of four Tesla;
• Figure 7 is a graph showing a field uniformity for a 1.5 Tesla magnetic field with an axial dimension for the electromagnet and utilizing an outer magnetic iron body
• Figure 8 is a graph similar to the graph of Figure 7 but showing field uniformity for a four (4) Tesla magnetic field;
• Figure 9 is a graph comparing the magnetic field intensity of the unitary superconducting electromagnet of this invention in which an outer magnetic iron body positioned about an inner body containing the coils, with an electromagnet having a twenty percent increase in the ampere turns of the coils, without an outer magnetic iron body.
• Figure 10 is a graph showing the permeability of a specific iron material from a magnetic intensity of one (1)
• Figure 11 is a graph showing the permeability of the iron material illustrated in Figure 10 from a magnetic intensity of two (2) Tesla to twelve (12) Tesla.
• Unitary electromagnet structure 10 is particularly adapted for use with nuclear magnetic resonance (NMR) equipment for medical apparatus.
• NMR nuclear magnetic resonance
• the present invention may be utilized in many applications, such as in accelerators, particle beams, spectroscopy, and the like.
• Unitary electromagnet structure 10 includes an inner cylindrical body or housing generally indicated at 12 and being of a toroidal shape having a central bore 14 therethrough and mounted within an outer magnetic iron body indicated generally at 16.
• Outer magnetic iron body 16 includes opposed end plate assemblies 16 between which inner body 12 is mounted.
• Bore 14 may be of a size sufficient to receive the body of a patient in the event the * electromagnetic structure 10 is used with NMR imaging.
• a cradle type base support generally indicated at 20 includes opposed end support cradle members 22 formed of a non-magnetic material with longitudinally extending beams 24 secured between end support members 22.
• cryogenic cooling means mounted on the upper surface of magnetic iron body 16 including a cooling medium, such as helium or nitrogen, for cooling the superconducting electromagnet 10 to the superconducting temperature of the materials used in the electromagnet.
• a cooling medium such as helium or nitrogen
• An inlet is shown at 28 for providing the cooling fluid about a fluid passageway for cooling the electromagnet and an outlet is shown at 30 for receiving the cooling fluid after cooling of the superconducting materials.
• Suitable cryogenic cooling means for providing such a cooling medium is well known in the art.
• inner cylindrical body 12 has an inner shell 32 which defines bore 14 and an outer concentric shell 34.
• An end ring 33. is secured to outer shell 34 at each end thereof.
• End plates 35 connect shells 32 and 34 at their ends to form a vacuum jacket for the superconducting coils.
• Forming an enclosed housing 36 for the superconducting coils are intermediate spaced shells 37 and 38 with closed ends 39 and providing an annular space therebetween receiving the superconducting coil assembly or coil pack indicated generally at 40 and between end plates 41.
• Coil pack 40 includes a main center coil 42 and an end coil 44 adjacent each end of center coil 42 separated by annular spacer plates 46 formed of a non-magnetic material.
• the wire from which coils 42 and 44 is formed is wound on a core or bobbin 47 supporting the wire within an accuracy of one (1) mil, and while tensioned under a predetermined tension dependent on the properties and size of the wire material.
• the wire preferably has a rectangular cross section having transverse dimensions of fifty (50) mils by one hundred (100) mils, for example, which includes the superconducting material.
• the wire is tensioned under a predetermined tension of around one hundred (100) pounds while being wound onto the associated supporting core 47 and the tension is gradually and progressively decreased during winding in accordance with precise calculations based on the type and size of materials utilized, the thermal and mechanical properties of the materials, and the position of the wire on the core.
• the use of a plurality of magnetic coils permits currents of different magnitudes to be utilized thereby to aid in obtaining a desired shaping and intensity of the magnetic field.
• the core or bobbin 47 is formed of a plurality of circumferentially spaced strips or slats 48 of around one- half (1/2) inch in width positioned about both the inner and outer surfaces of coils 42 and 44 and forming flow passages between strips 48 to permit the flow of cooling fluid along coils 42 and 44.
• a layer of stainless steel banding 52 is wrapped around the outer strips 48 as shown in Figure 5. As shown particularly in Figure 5, helium from inlet 28 flows to a fluid passage 49 adjacent coil pack 40 and then through openings 50 in end plates 41 and spacer plates 46 along coil pack 40 and the spacings between strips 48 to fluid passage 51 for return to the cryogenic cooling means 26 through outlet 30.
• An inner heat shield shown generally at 54 of a metallic material for a temperature of twenty (20) degrees Kelvin and segmented to minimize eddy currents is formed about housing 36 for enclosing coil pack 40.
• An outer heat shield 56 extends about inner heat shield 54 in spaced relation thereto and is formed of a metallic material for a temperature of eighty (80) degrees Kelvin and is segmented to minimize eddy currents.
• a super insulating material is provided in the space shown at 58 between outer heat shield 56 and the vacuum jacket formed by inner and outer shells 32, 34.
• means are provided for exerting a vacuum in the space between shells 32, 34, and for providing a cooling fluid for cooling heat shields 54, 56.
• Outer magnetic iron body 16 includes a plurality of longitudinally extending sections indicated at 60.
• Each section 60 has a plurality of plates secured to each other and forming laminations extending between end plate assemblies 15.
• Each section 60 has secured adjacent each end thereof a radially extending segment 62 secured by bolts 64 to an end ring 66 thereby to form the end plate assembly indicated at 18 positioned adjacent each end of inner cylindrical body 12.
• the contacting sides 68 of adjacent segments 62 have a layer of insulating material thereon, such as insulating paint. By having segments 62 insulated from each other, this breaks up the paths of the eddy current by increasing the resistance to such paths. Also, if desired, segments 62 may be formed of a plurality of connected plates which break up the paths of the eddy currents.
• end plate assemblies 15 particularly as shown in Figure 4 are positioned closely adjacent the end of inner body 12 and coil pack 40, and have inner and outer diameters generally corresponding to those of inner body 12.
• Outer body 16 provides a closed magnetic path extending from coil pack 40 along end assemblies 18 and connecting sections 60 of outer iron body 16.
• the portions of end assemblies 18 radially inward of line 69 shown in Figure 4 when positioned closely adjacent coil pack 40 have a magnetic intensity greater than the saturation of iron or above around 1.5 or 1.7 Tesla while the remainder of outer body 16 including longitudinally extending sections 60 having a magnetic intensity at the saturation level.
• the magnetic intensity of end assemblies 18 radially inward of line 69 would vary from around 1.5 Tesla to around four (4) Tesla.
• end assemblies 15 which form pole faces aid in increasing the intensity of the magnetic field.
• accurate information concerning the thermal and magnetic properties of the iron in end assemblies 18 is required for predicting precisely the permeability of the iron at all values of the magnetic field. Up to around 2.5 Tesla, the permeability is measured in accordance with well known techniques.
• adjusting blocks 70 are secured to the outer surface of ring 33 on outer shell 34 of inner body 12 at spaced intervals of one hundred twenty (120) degrees about the outer circumference of outer shell 34 and adjacent each end thereof to provide three (3) longitudinally aligned pairs of adjusting blocks 70 which fit within suitable slots 71 provided in adjacent sections 60 of outer magnetic iron body 16.
• Blocks 72 and 74 contain adjustable set screws 76 accessible to workmen through the space between sections 60.
• Set screws 76 have their inner ends in contact with blocks 70 for moving inner cylindrical body 12 within radial and axial distances of one (1) mil relative to outer magnetic iron body 16 through suitable adjustment of screws 76 on blocks 72 and 74.
• a high quality magnetic field homogeneity or uniformity of at least one (l) part per million is required for many applications of the unitary superconducting electromagnet and some applications, such as spectroscopy require a uniformity in the magnetic field of one (1) part per one hundred million.
• a very precise method or series of steps is involved in the design of the electromagnet. First, the volume, shape and intensity of the magnetic field for the superconducting electromagnet are established and an initial design is created for maximizing field intensity with a minimal coil size and including some means such as a central bore for access to the magnetic field in order to place objects therein for testing or treatment.
• the initial design includes positioning the superconducting coils and outer ferromagnetic body in relation to each other and to the magnetic field so that the desired effect on the intensity and shape of the magnetic field may be obtained from radial and axial adjustment of the coils and outer magnetic iron body in relation to each other within a tolerance of one (1) mil.
• the initial design is established based on one (1) computer programs using (a) parameters of the volume, shape, and intensity of the desired magnetic field; and (b) accurate information concerning the mechanical, thermal, electrical, and magnetic properties of the materials proposed for the unitary electromagnet, including particularly the thermal and magnetic properties of the iron for predicting precisely the permeability of the iron at all values of the magnetic field; (2) appropriate engineering techniques to obtain the precise positioning of the components of the magnet including particularly the positioning of the inner body coils and the magnetic iron body in relation to each other and to the magnetic field; and (3) utilization of appropriate electromagnetic equations for achieving a practical, economical, engineering design.
• the computer programs may incorporate, if desired, the appropriate engineering techniques and electromagnet equations set forth under items (2) and (3) above or these items may be employed separately from the computer programming in obtaining the initial design.
• a computer program which has been utilized satisfactorily in the design of the present invention is referred to as the "Magnus" program and is set forth in the following publications authored by Sergio Pissanetzky and obtainable from Texas Accelerator Center, 2319 Timberloch Place, The Woodlands, Texas 77380 U.S.A.:
• the magnetic iron of the outer body increases the intensity of the magnetic field at least around ten (10) percent and as much as around twenty (20) percent or more due to the precise location of the magnetic iron with respect to the magnetic field and the coils.
• the outer magnetic iron body absorbs and directs the magnetic fringe field to provide such an increase in the magnetic intensity. This is accomplished by a distribution of the flux in a uniform manner and at magnetic field intensities substantially above the magnetic saturation of the magnetic iron as high as around ten (10) Tesla.
• the accurate and precise positioning of the outer magnetic iron body relative to the coils within a tolerance of one (1) mil also results in the high degree of uniformity of the field of at least around one (1) part per million.
• Figures 6-9 are directed to graphs which illustrate the results obtained for a superconducting unitary electromagnet constructed in accordance with the foregoing method and as shown and described in the drawings.
• a graph is illustrated showing a magnetic fringe field at four (4) Tesla.
• the R direction extends radially from the center of central bore 14 which defines the working volume of electromagnet 10 and the C direction in centimeters is along the longitudinal axis defined by bore 14.
• a five (5) gauss line and a ten (10) gauss line are shown extending outwardly from the bore of electromagnet 10.
• Figure 7 shows the uniformity along the Z axis or longitudinal centerline of bore 14 for an intensity of 1.5 Tesla in a persistent mode. It is noted that a very high uniformity is obtained of + ten (10) ppm within 8.1 cm without the use of any trim coils.
• a graph is shown illustrating the homogeneity for a magnetic field intensity at four (4) Tesla in an axial direction along the longitudinal centerline of bore 14. It is noted a uniformity of twenty (20) ppm is obtained within eleven (11) cm without the use of any trim coils or trim fields.
• this graph shows the increase in magnetic field intensity of over twenty (20) percent with the utilization of a magnetic iron body about the superconducting coils with the increase in intensity being generally uniform for field intensities from one-half
• Curve A illustrates an electromagnet in which magnetic coils are utilized without a magnetic iron body with the coils being wound with twenty (20) percent additional ampere turns.
• Curve B indicates the field quality obtained with a unitary superconducting electromagnetic structure in accordance with the present invention in which an magnetic iron body about the coils is utilized.
• FIG. 10 shows permeability for a magnetic intensity to two (2) Tesla
• Figure 11 shows the permeability for a magnetic intensity from two (2) Tesla to twelve (12) Tesla. It is noted from Figure 11 that the permeability of air is one and the permeability from four (4) Tesla to twelve (12) Tesla is between one and two. The permeability was calculated in accordance with the aforementioned publication entitled "The Interpolation of Magnetization Tables".
• the present invention is particularly adapted for use with magnetic fields having an intensity greater than the magnetic saturation of iron, it is to be understood that the present invention may be utilized with magnetic field intensities less than the magnetic saturation of iron.
• the embodiment of the electromagnet illustrated herein utilizes means for adjusting the position of the coils relative to the outer iron body after construction of the electromagnet, it is to be understood that some embodiments of this invention may have the coils and outer iron body fixed in relation to each other after construction thereby requiring a highly accurate positioning of such coils and outer iron body in the design being constructed or manufactured.

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• 1987
  • 1987-08-14 US US07/085,546 patent/US4822772A/en not_active Expired - Lifetime
• 1988
  • 1988-07-27 KR KR1019890700607A patent/KR920008203B1/ko not_active IP Right Cessation
  • 1988-07-27 WO PCT/US1988/002543 patent/WO1989001696A1/fr not_active Application Discontinuation
  • 1988-07-27 JP JP63508071A patent/JP2643402B2/ja not_active Expired - Lifetime
  • 1988-07-27 EP EP19880908855 patent/EP0374189A4/en not_active Withdrawn
  • 1988-07-27 AU AU25319/88A patent/AU2531988A/en not_active Abandoned
  • 1988-08-10 IL IL87407A patent/IL87407A/xx not_active IP Right Cessation
  • 1988-08-12 PH PH37407A patent/PH24819A/en unknown
  • 1988-08-12 MX MX012657A patent/MX165621B/es unknown
  • 1988-08-15 CA CA000574750A patent/CA1317630C/fr not_active Expired - Fee Related

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